The present invention relates to cross-channel interference and in particular, though not necessarily, to the measurement of cross-channel interference levels in high frequency receivers.
In many multi-channel electrical communication systems operating at radio or microwave frequencies, a significant source of noise in a given channel arises from cross-channel interference, i.e. due to the intermodulation of the signals carried by two or more other channels. In order to optimise system performance, designers seek to measure cross-channel interference levels as a step towards reducing interference.
Considering for example a cable television receiver which is capable of receiving many individual television channels, the design stage of the receiver typically involves measuring the noise level in each channel due to cross-channel interference. This is carried out using an array of signal generators corresponding in number to the number of channels. Each signal generator is set to generate a signal which is centred upon the carrier frequency of a corresponding television channel. The output of the signal generators are combined and fed to an input of the receiver.
Measuring equipment, e.g. a spectrum analyser, is connected to an output of the receiver and is tuned in turn to each of the channel carrier frequencies. For each channel, the signal generator responsible for generating the carrier frequency of that channel is switched off, such that the measured signal corresponds substantially to the cross-channel interference power level. The output of the signal generators may be modulated to some extent to simulate the information component which would normally be carried by the channel, i.e. a television picture. In order to isolate the channels chiefly responsible for cross-channel interference in a given channel, signal generators may be switched on and off in turn.
It will be appreciated that, particularly for a system designed to handle a large number of channels, the above test system will require an extremely large number of signal generators. For example, testing of a modem cable television receiver may require the use of over a hundred signal generators. Such a test system is complex and expensive to implement. Furthermore, the process of isolating those channels which are responsible for cross-channel interference at any given channel, requiring as it does the switching on and off of signal generators, is time consuming and prone to error.
It is an object of the present invention to overcome or at least mitigate the disadvantages of known cross-channel interference test systems. It is also an object of the present invention to provide a cross-channel interference test system which requires only a relatively small number of signal generators to simulate cross-channel interference effects.
According to a first aspect of the present invention there is provided, in a multi-channel electrical communication system, a method of estimating the interference level in a test channel due to the intermodulation of two or more other channels, where each channel is centred upon a carrier frequency, the method comprising, for a given m-th order mode:
identifying the set(s) of carrier frequencies, the m-th order linear combination of which gives rise substantially to the carrier frequency of said test channel; and
for the or each of the identified set of carrier frequencies, tuning signal generators to respective ones of the carrier frequencies, applying the generated signals to an input of the communication system, and measuring at an output of the system the intermodulation interference at the carrier frequency of the test channel.
Where a plurality of sets of carrier frequencies are identified as combining to generate intermodulation interference in the test channel, the same signal generators are used for each set, and are merely retuned to the necessary frequencies. The total interference level for the mode in question may be determined by adding together the measured result for each set.
The present invention allows the total intermodulation interference which will occur in a given channel to be predicted using only a relatively small number of signal generators, i.e. the number of signal generators corresponds to the intermodulation mode under investigation. Rather than providing a signal generator for each channel, signal generators are re-used to determine the intermodulation interference for each set of carrier frequencies which are predicted to cause interference at the test channel. It is generally only necessary to measure intermodulation interference in a multi-channel system for relatively low order modes. For example, it may be sufficient to measure intermodulation interference for only the second and third modes. For the second mode, using the present invention, only two signal generators are required, whilst for the third mode three signal generators are required. This is in contrast to conventional test processes where the number of signal generators required corresponds to the total number of channels (or one less than the total number of channels).
Preferably, for the or each identified set of carrier frequencies, it is determined whether or not the frequencies of the set will give rise to inter-modulation interference at modes higher or lower than the mode in question, i.e. m. If such higher and/or lower mode interference is likely to arise, the output of two or more of the signal generators may be offset from the respective carrier frequencies such that the higher and/or lower mode intermodulation interference is shifted away from the carrier frequency of the test channel whilst that due to the m-th order intermodulation remains at the carrier frequency of the test channel.
It is possible to predict the change in the intermodulation interference power level at the test channel which will result from a given change in the power level of one or more of the channels contributing to the interference. Using suitable power relationships, the invention may be applied to predict the interference power level which will result from a set of channels having respective power levels, by measuring the interference power levels which result from the same set of channels having different power levels. The measured output powers are normalised, in accordance with the power relationships, to provide the necessary prediction.
Similarly, normalisation may be applied to the measured output powers in order to take account of a non-uniform frequency response. For example, where the input channels have a uniform power level, the measured output powers may be normalised to simulate the effect of non-uniform power levels, e.g. due to the frequency response of a signal transmission medium.
Preferably, the method of the present invention is repeated for each intermodulation interference mode which is expected to contribute significantly to cross-channel interference.
According to a second aspect of the present invention there is provided apparatus for testing a multi-channel communication system to estimate the interference level in a test channel due to the intermodulation of two or more other channels, where each channel is centred upon a carrier frequency, the apparatus comprising:
processing means for identifying the set(s) of carrier frequencies, the m-th order linear combination of which gives rise substantially to the carrier frequency of said test channel;
signal generators arranged to be tuned to respective ones of identified carrier frequencies of the or each set to provide output signals to an input of a communication system under test; and
measuring means for measuring the output of said system under test to determine the inter-modulation interference at the carrier frequency of the test channel.
Where a plurality of sets of carrier frequencies are identified by the processing means, the signal generators are arranged to be tuned to the first set of frequencies, followed by the second set, etc. The measuring means is preferably arranged to add the measured outputs for each set of frequencies.
Preferably, the processing means is provided by a computer, the computer further being arranged to control the signal generators. The computer may also provide the measuring means.